You missed what kanewolf was getting at. If one fan of a new set of three is running slower than the others or even STOPPPING, that FAN is faulty. There is NOT likely a problem with the mobo fan header. You CAN test that using another mobo fan header, but you'd have to ensure that the speed control signal it is sent from the original and alternative the headers is the SAME. It does appear very suspicious to me that the slow or stopped performance is apparent when your system is hotter and the other fans are running fast -that further indicates a faulty fan.